There has been an approach to decrease rolling resistance of a tire to suppress the heat build-up and thus improve fuel economy of a vehicle. The demand for improving fuel economy of a vehicle by means of tire improvement has increased in recent years. In order to improve fuel economy, tread and sidewall improvements have been highly required. Still the demand has been increasing year by year, and other components such as a breaker are required to achieve improvement in fuel economy. Also, tires for high-load vehicles, such as trucks and buses, as well as tires for passenger vehicles are required to have higher fuel economy.
Examples of known methods for achieving low heat build-up of rubber compositions include a method using a low-reinforcing filler, and a method using a smaller amount of reinforcing filler. However, such methods for improving fuel economy using a filler cause reduction in the effect of reinforcing the rubber composition, and thereby problematically cause reduction in breaking performance. Hence, it has been generally difficult to achieve both high levels of fuel economy and breaking performance.
Natural rubber, which has been commonly used for breaker rubbers, has a higher Mooney viscosity than those of synthetic rubbers and thus has low processability. Therefore, natural rubber to be used is usually added with a peptizer and then masticated so that the rubber has a decreased Mooney viscosity. Thus, use of natural rubber decreases productivity. Further, the mastication causes molecular chain scission in natural rubber, thereby problematically leading to a loss of the properties (e.g., rubber strength) of high-molecular-weight polymer which natural rubber essentially has.
Patent Document 1 discloses a rubber composition containing natural rubber that has been deproteinized so as to have a total nitrogen content of 0.1% by mass or less as an index of protein content, which achieves favorable levels of strength, low heat build-up, and processability. However, the rubber composition still has room for improving both fuel economy and breaking performance while simultaneously securing processability.    Patent Document 1: JP H06-329838 A